1. Field of the Invention
The present invention relates to an imaging device for use in an electrography or electrographic imaging process. The imaging device does not use a conventional discrete charging device and cleaning unit, and accordingly has a simplified and compact structure and does not cause an environmental pollution problem. The imaging device can be advantageously used as, or in, a copier or copying machine, printer, facsimile device and other image-providing devices. The term "electrography" used herein means that it includes any image formation processes wherein a latent image is first produced on an image-carrying element and then it is developed with a developer to form the corresponding visible image, for example, electrophotography, xerography and the like.
2. Description of the Related Art
The conventional electrographic imaging device uses the series of process steps shown in FIG. 1. Namely, the process steps include:
(a) image exposure; PA1 (b) development; PA1 (c) image transfer; PA1 (d) cleaning of the residual toners; and PA1 (e) pre-charging; PA1 (a) image exposure; PA1 (b) development of a latent image and recovery of the residual toners; PA1 (c) image transfer; PA1 (d) distribution of the residual toners; and PA1 (e) pre-charging. PA1 an image-carrying element; PA1 an image exposure means for forming a charged latent image on a surface of the image-carrying element; PA1 a developing means for forming a visible image on the surface of the image-carrying element as a function of development of said latent image with a developer and, at substantially the same time, recovering a residual developer, not transferred to an image-receiving element in the preceding image transfer process, from the image-carrying element; PA1 an image transfer means for transferring the visible developer image from the image-carrying element to an image-receiving element; and PA1 a distribution roller means of an elastic and electrically conductive material for uniformly redistributing the residual developer, not transferred to the image-receiving element in the above image transfer process, over the surface of the image-carrying element and, at substantially the same time, deelectrifying the charged surface of the image-carrying element with an AC electric field and, in combination with an application of a DC voltage, freshly charging the deelectrified surface of the image-carrying element; PA1 with the proviso that a cleaning means for removing the residual developer, not transferred to the image-receiving element, from the image-carrying element is not located between said image transfer means and said image exposure means. PA1 an optical image-providing system, selected from the group consisting of LED (light emitting diode), laser, liquid crystal shutter and EL (electroluminescence) optical systems, as said image exposure means; PA1 a developing device including a development sleeve and a magnet roller as said developing means; PA1 a transfer roller as said image transfer means; and PA1 an electrically conductive roller of an elastic material containing closed cells as said distribution roller means.
and these steps are repeated. In the image exposure step, an image-carrying element such as a photosensitive drum is exposed to image-forming light, i.e., light image, in an optical unit such as a LED array, to form a charged latent image after it has been sensitized by electrical charging in a preceding pre-charging step. The latent image is formed as a function of a photoconductive discharge of the electrically charged surface of the photosensitive drum. The formed latent image is physically developed using a toner or toning agent as a developer in a developing device. A visible image of the toner is formed on the drum surface as a result of an electrical attraction of fine particles of the toner thereon. The developed image of the toner is then transferred to an image-receiving element such as paper, and the transferred toner image is fixed thereon by fusing. In this image transfer step, some small amount of the toner remains on the surface of the photosensitive drum without being transferred to the paper, and it can adversely affect on the results of the subsequent imaging process if it is not removed from the drum. It is therefore essential in the conventional imaging process to remove the residual toner from the drum in a cleaning step, prior to reusing the drum for the next imaging process. After cleaning thereof, the drum is again sensitized, by electrical charging, in a pre-charging step.
FIG. 2 illustrates a typical example of the electrographic imaging device suitable for carrying out the above-described imaging process. A photosensitive drum 102 which is rotatable in the direction shown by an arrow is electrostatically charged by a pre-charging device 101 such as a scorotron charger. The drum 102 is then exposed to a light image from a LED array 103 which can emit light depending upon a predetermined imaging signal, thereby forming a charged latent image on the drum surface. The latent image is developed with fine particles of a developer or toner 105 in a developing device 104. A visible image of the toner is thus formed on the drum surface. The developed image is then guided to an image transfer station provided with an image transfer device 107, and is transferred to a paper 106 supplied from a paper cassette (not shown). The transferred image is fused and fixed to the paper 106 by means of a pair of fixing rollers 39. Since the drum 102 has a residual toner on a surface thereof, it is then cleaned in a cleaner 108. After removal of the residual toner from a surface thereof, the drum 102 is again charged by the pre-charging device 101 for reuse in the next imaging process.
The above-described imaging process and device, however, suffer from several drawbacks. The use of the cleaner is a bar to providing a compact and low price device, since it becomes necessary to use additional devices such as a device for disposing of the toner recovered in the cleaner and a device for storage the disposed toner. Alternatively, if the disposed toner is intended to be reused in the developing device, it becomes necessary to add an additional and expensive device for conveying the toner from the toner disposal device to the developing device. Further, disposal of the residual toner causes an environmental problem and increases the operation cost.
It has been found that the problems due to use of the cleaner can be solved if the imaging device uses the cleanerless imaging process, a flowchart of which is shown in FIG. 3, disclosed in U.S. patent application Ser. No. 220,208, filed on Mar. 31, 1994, which has been assigned to the applicant.
The cleanerless imaging process, as shown in FIG. 3, includes the following process steps:
Comparing the process of FIG. 3 with that of FIG. 1, it will be appreciated that the cleanerless imaging process is distinguished from the conventional imaging process, because the residual toner is recovered in the development step and accordingly a cleaning step is omitted from the imaging process. Further, in order to assist in recovering the residual toner in the development step, the process includes a distribution step for the residual toner prior to pre-charging of the photosensitive drum. In this distribution step, the residual toner remaining on the surface of the drum after the image transfer step is uniformly distributed over the surface of the drum.
FIG. 4 illustrates a typical example of the imaging device for carrying out the above-described cleanerless imaging process. Note that, in order to simplify a comparison of the device of FIG. 4 with that of FIG. 2, the same reference numerals are assigned to the same devices, means or elements.
In the imaging device of FIG. 4, as described in detail in the above-cited U.S. Ser. No. 208, a developing device 104 can act as a recovery means for recovering the residual toner from the surface of the photosensitive drum 102, and as a developing means for the charged latent image formed as a function of image exposure in a LED array 103.
Positioned between an image transfer device 107 wherein the developed toner image is transferred from the drum 102 to a paper 106, and a pre-charging device 101 wherein the drum 102 is sensitized with electrical charging, is a distribution means 110 in the form of a rotatable roller. The distribution means 110 is preferably a conductive roller made of a foamed plastic material such as polyurethane, and is rotated in contact with the photosensitive drum 102 bearing the residual or not-transferred toner on a surface thereof. During the rotation thereof, the distribution means 110 can uniformly redistribute the patternwise distributed residual toner over the surface of the drum to thereby reduce the amount of the residual toner per the area of the drum surface. Such a uniform distribution of the residual toner can be further assisted by vibration of the distribution means 110 caused by an application of an AC voltage to said means 110 by means of an AC electric source 111. The application of the AC voltage is also effective to remove charge, i.e., deelectrify the charged surface of the drum 102, i.e., remove the remaining electric charge from the surface of the drum 102. The deelectrification of the drum surface will ensure a recovery of the residual toner in the developing device 104, because it can reduce the electrical attraction between the toner and the drum surface.
Although it has many important advantages, the prior art imaging device still suffers from some problems. First, since it is necessary to use the distribution means and the pre-charging device in combination, the imaging device cannot satisfy the requirements for a further reduction in size and for a further reduction in the production cost of the device. Second, a high voltage electric source must be used in combination with the pre-charging device. It is desired to avoid using such high voltage electric source, because it can generate ozone as an undesirable gas, in addition to increasing the size and cost of the device. Third, since small amounts of the paper dust can be conveyed into the developing device and shorten the life of the device, it is necessary to additionally locate a suitable dust removing means between the image transfer device and the developing device, however, such dust removing means also is a bar to providing a compact device.